This invention relates to rotor blades for rotorcraft such as helicopters and the like, and more particularly to an improved construction and control scheme for such rotor blades which permits a significant reduction in noise generated by the blades.
Conventional helicopters in low speed descent flight conditions frequently generate an impulsive noise signature which is commonly referred to as blade-vortex interactions (BVI) noise or xe2x80x9cblade slapxe2x80x9d. BVI noise is generated by blade tip vortices, which interact with the rotor blades. Unfortunately, it is typically within a frequency range where human subjective response to noise is high, and because of its high harmonic levels in this frequency range, BVI noise has been identified as one of the most annoying, or objectionable, sounds produced by a helicopter. Consequently, a reduction in the BVI noise intensity and changes in the noise signature, using active and/or passive noise control techniques, is desirable to the rotorcraft industry, which is challenged by today""s stringent military and civilian acoustic regulations.
Fundamentally, BVI noise is generated as a result of large temporal fluctuations in the blade aerodynamic loads due to close encounters with elements of the tip vortex wake. These unsteady loads are also known to result in an increase in the rotor vibration levels and the attendant decrease in the fatigue life of rotor dynamic components. An active control method that reduces the rotor vibration levels for all flight conditions is therefore highly desired.
Three vortex-related parameters are known to affect the intensity of BVI noise and rotor vibration levels. These parameters are:
a) tip vortex strengthxe2x80x94this parameter is directly proportional to the magnitude of the induced aerodynamic loads. The intensity of BVI noise and rotor vibration levels are also directly related to the magnitude of the induced aerodynamic loads;
b) the intensity of BVI noise and rotor vibration levels are inversely proportional to the blade/vortex separation distance;
c) vortex orientation with respect to the bladexe2x80x94specifically whether the vortex is parallel (induces the largest temporal aerodynamic load variations), oblique, or perpendicular (not a concern for BVI noise but will have some impact on rotor vibration).
The objective in reducing BVI is to decrease the adverse effect of the interaction between the rotor blade and the tip vortices. Existing devices which have been used for reducing BVI noise include Higher Harmonic blade pitch control (HHC), which seeks to change the blade tip vortex strength, and thus the local aerodynamic conditions, through blade pitch changes. Other control means concentrate primarily on reducing the strength of the tip vortex through blade tip geometric modifications. Typical examples are the use of leading and trailing edge sweep, the use of blade anhedral, and the use of a subwing. All of these examples, excluding HHC, may be classified as passive control techniques. An example of another active control technique would be the use of tip air mass injection, which again has the purpose of weakening the blade tip vortices.
Each of the foregoing prior art solutions to BVI noise has been at least partially unsuccessful, either because of ineffectiveness or because of the solution""s detrimental side effects with respect to the flight characteristics and efficiency of the rotorcraft. For example, HHC methods change the aerodynamic conditions along the entire blade in order to reduce BVI noise, due to the change in blade pitch. Passive BVI noise control methods are not adaptable to changing BVI conditions throughout the flight regime, which are associated with changes in aircraft descent rate and forward flight speed. Additionally, most of the prior art passive solutions to the BVI problem are deployed at all times, whether or not needed, often degrading flight performance unnecessarily.
More recently, the assignee of the present invention developed improved systems for reducing BVI noise, which are the subject of U.S. Pat. Nos. 5,588,800 (the ""800 patent) and 5,711,651 (the ""651 patent), herein expressly incorporated by reference. These patents disclose an active control device and system, comprising trailing edge flaps disposed near the tip of each of a rotorcraft""s rotor blades. The flaps may be actuated in any conventional way (such as hydraulically, electrically, electromagnetically, pneumatically, etc.), and may be scheduled to be actuated to a deflected position during rotation of the rotor blade through predetermined regions of the rotor azimuth, and further scheduled to be actuated to a retracted position through remaining regions of the rotor azimuth. Through the careful azimuth-dependent deployment and retraction of the flap over the rotor disk, using scheduling principles developed by the patentees, blade tip vortices are a) made weaker, and b) pushed farther away from the rotor disk (so that larger blade-vortex separation distances are achieved).
The ""800 and ""651 patents, for example, disclose the use of one-piece, constant chord, integral-type trailing edge flaps, as well as multi-segment flaps. For a rigid model rotor, experiments have shown that a blade-mounted trailing edge flap can be used to alleviate rotor BVI noise and vibration levels using nonharmonic and harmonic flap schedules, respectively. For a full scale elastic blade, numerical studies have indicated reductions in BVI noise levels can be achieved by harmonic actuation of flaps causing increased blade/vortex separation distance in response to altering the blade spanwise pitch distribution and hence the aerodynamic loading. The use of Higher Harmonic root pitch control has also very similar effects to those obtained with the use of a trailing edge flap on an elastic blade. The use of Tip Air Mass Injection is an alternate example in which a high energy air jet is introduced at the tip of the blade and aimed towards the center, or the core, of the tip vortex with the intent of diffusing (or weakening) its strength. The use of continuous blowing/suction to control the unsteady aerodynamic response of a rotor blade during BVI were recently demonstrated numerically (and is the subject of U.S. Pat. No. 5,813,625).
Passive control systems, such as those based on varying the blade leading and trailing edge spanwise sweep angle distribution, blade anhedral, spoilers, and a subwing, attempt to impact one, two or three of the above mentioned factors that influence the intensity of BVI. A clear disadvantage of the use of passive control means, as contrasted to active control means, for the alleviation of rotor BVI noise and vibration reduction is their inability to adapt to other flight conditions. Consequently, once these features are permanently integrated into the design of a blade, rotor aerodynamic performance can deteriorate and noise and vibration levels can exceed the acceptable levels at conditions other than those representative of the design low-speed descent flight conditions.
This invention further refines and improves the innovative systems first taught in the aforementioned ""800 and ""651 patents.
More particularly, there is provided an active control device for reducing undesirable effects generated by a rotorcraft having a rotor blade including a tip end, a root end, a leading edge, and a trailing edge. As is conventional, the rotor blade is attached at its root end to a rotor hub on the rotorcraft, and extends radially outwardly therefrom, having a radius R and a chord C, such that when the rotor hub is rotatably driven, the rotor blade rotates about the hub through a 360 degree azimuth. The active control device comprises a movable flap comprised of a plurality of segments (preferably from two to four, though any number may be employed) which are each pivotally attached to the rotor blade trailing edge so that they each may be selectively independently deflected in both positive and negative directions (downwards and upwards, respectively, relative to the rotor blade surface). Advantageously, a radially outer one of the flap segments has a smaller chord length than a radially inner one of the flap segments. More specifically, the movable flap is tapered, such that each successive flap segment in a radially outward direction has a smaller chord length than an immediately preceding flap segment. In practice, this means that, of the aforementioned plurality of flap segments, a first radially innermost flap segment has a first chord length, and a second flap segment disposed radially outwardly of the first flap segment has a second chord length which is smaller than the first chord length. Furthermore, a third flap segment disposed radially outwardly of the second flap segment has a third chord length which is smaller than the second chord length. A fourth flap segment disposed radially outwardly of the third flap segment has a fourth chord length which is smaller than the third chord length.
The rotor blade may comprise a conventional helicopter rotor blade, or alternatively, a proprotor blade having a high degree of twist.
In another aspect of the invention, there is provided an active control device for reducing undesirable effects generated by a rotorcraft, which has a rotor blade including a tip end, a root end, a leading edge, and a trailing edge. The rotor blade is attached at the root end to a rotor hub on the rotorcraft and extends radially outwardly therefrom, having a radius R and a chord C, such that when the rotor hub is rotatably driven, the rotor blade rotates about the hub through a 360 degree azimuth. The active control device comprises a movable flap which is pivotally attached to the rotor blade trailing edge so that it may be selectively deflected in both positive and negative directions. The movable flap has a first chord length at a first radially inner location and a second chord length which is smaller than the first chord length at a second radial location which is outwardly of the first radial location. Preferably, the movable flap comprises a plurality of segments which are independently controllable to deflect individual ones of the segments in the aforementioned positive and negative directions.
In still another aspect of the invention, there is taught a unique method for reducing blade vortex-interaction (BVI) noise generated by a rotorcraft having a rotor blade including a tip end, a root end, a leading edge, and a trailing edge, wherein the rotor blade is attached at the root end to a rotor hub on the rotorcraft and extends radially outwardly therefrom, such that when the rotor hub is rotatably driven, the rotor blade rotates about the hub through a 360 degree azimuth, the rotor blade further comprising a movable flap comprising a plurality of independently controllable segments pivotally attached to the trailing edge, and arranged radially therealong. The method comprises the steps of actuating a first one of the plurality of flap segments to a first predetermined flap deflection angle at a particular predetermined rotor blade azimuth, and actuating a second one of the plurality of flap segments to a second predetermined flap deflection angle at the particular predetermined rotor blade azimuth. Advantageously, when the rotor blade azimuth is between a range of about 1 and about 115 degrees, both of the first and second flap deflection angles are positive, and the first flap deflection angle is greater than the second flap deflection angle. Furthermore, when the rotor blade azimuth is between a range of about 165 and about 310 degrees, both of the first and second flap deflection angles are negative, and the first flap deflection angle is greater than the second flap deflection angle. Preferably, the aforementioned first flap segment is disposed radially inwardly of the second flap segment, and has a larger chord length than the second flap segment. The respective flap segments may be deflected out-of-phase with one another, or in-phase relative to one another, as shown in FIGS. 2 and 3.
In yet another aspect of the invention, there is disclosed a method for reducing blade vortex-interaction (BVI) noise generated by a rotorcraft having a rotor blade including a tip end, a root end, a leading edge, and a trailing edge, wherein the rotor blade is attached at the root end to a rotor hub on the rotorcraft and extends radially outwardly therefrom, such that when the rotor hub is rotatably driven, the rotor blade rotates about the hub through a 360 degree azimuth. The rotor blade further comprises a movable flap comprising a plurality of independently controllable segments pivotally attached to the trailing edge, and arranged radially therealong. The inventive method comprises the steps of deflecting one of the flap segments to a negative deflection angle and deflecting another of the flap segments to a positive deflection angle.
In still another aspect of the invention, there is disclosed a method for reducing blade vortex-interaction (BVI) noise generated by a rotorcraft having a rotor blade including a tip end, a root end, a leading edge, and a trailing edge, wherein the rotor blade is attached at the root end to a rotor hub on the rotorcraft and extends radially outwardly therefrom, such that when the rotor hub is rotatably driven, the rotor blade rotates about the hub through a 360 degree azimuth. The rotor blade further comprises a movable flap comprising a plurality of independently controllable segments pivotally attached to the trailing edge, and arranged radially therealong. The method comprises the step of deflecting radially outer ones of the flap segments to desired flap deflection angles, and maintaining radially inner ones of the flap segments in a neutral (undeflected) orientation.
In another aspect of the invention, there is disclosed a method for reducing rotor blade vibration generated by a rotorcraft having a rotor blade including a tip end, a root end, a leading edge, and a trailing edge, wherein the rotor blade is attached at the root end to a rotor hub on the rotorcraft and extends radially outwardly therefrom, such that when the rotor hub is rotatably driven, the rotor blade rotates about the hub through a 360 degree azimuth. The rotor blade further comprises a movable flap comprising a plurality of independently controllable segments pivotally attached to the trailing edge, and arranged radially therealong. The inventive method comprises the step of deflecting radially inner ones of the flap segments to desired flap deflection angles, and maintaining radially outer ones of the flap segments in a neutral (undeflected) orientation.
The invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying illustrative drawing.